This invention relates generally to hydraulic fracturing operations, and more specifically relates to the field of systems for real-time monitoring and control of downhole hydraulic fracturing operation in petroleum reservoirs.
Various fracture-stimulation techniques are designed and employed in the petroleum industry for the purpose of placing sand proppant in hydraulically induced fractures to enhance oil or gas flow through a reservoir to the wellbore. Hydraulic fracturing of petroleum reservoirs typically improves fluid flow to the wellbore, thus increasing production rates and ultimate recoverable reserves. A hydraulic fracture is created by injecting a fluid, such as a polymer gelled-water slurry with sand proppant, down the borehole and into the targeted reservoir interval at an injection rate and pressure sufficient to cause the reservoir rock within the selected depth interval to fracture in a vertical plane passing through the wellbore. A sand proppant is typically introduced into the fracturing fluid to prevent fracture closure after completion of the treatment and to optimize fracture conductivity.
A hydraulic fracturing treatment is a capital-intensive process. In addition to the substantial cost of a fracturing treatment itself, substantial oil and gas revenues may be gained as a result of a technically successful stimulation job, or lost due to an unsuccessful treatment. The effectiveness of a fracturing treatment depends on numerous critical design parameters, including reservoir rock properties, the vertical proximity of water-productive zones, and the presence or absence of strata that act as barriers. Unsuccessful fracturing treatments typically result from inefficient placement of sand proppant in the induced fracture with respect to the targeted reservoir interval, which also sometimes results in excessive water production due to treating "out of zone."
The formation is composed of rock layers, or strata, which include the objective petroleum reservoir, which is often a sandstone, limestone, or dolomite interval. When a fracture propagates vertically out of the defined hydrocarbon reservoir boundaries into adjacent water-productive zones, the well may be mined by excessive water flow into the wellbore, or added expenses and disposal problems may be caused by the need to safely dispose of the produced water. Also, if the fracture propagates into an adjacent non-productive formation, the sand proppant may be wasted in areas outside the objective formation, and the treatment may not be effective. Either situation may result in dire economic consequences to the well operator. Although it is sometimes possible to save a well that has been fractured "out of zone" such remedial efforts are typically extensive, risky, and costly.
An economical and successful fracture stimulation requires maximum controlled placement of fracture proppant in the reservoir zone, while avoiding treating into water-producing strata. The increased production revenue from successful fracturing treatments amounts to many millions of dollars each year. A successful fracturing treatment is typically evidenced by increased reservoir production performance resulting from concentrated placement of sand proppant in the petroleum reservoir within the induced hydraulic fracture.
Conversely, inefficient fracturing treatments cost the petroleum industry many millions of dollars each year both in foregone revenue from non-production of valuable hydrocarbons and in lost capital expenses associated with well drilling and completion. Indeed, some wells can be mined entirely from poor fracturing.
Known systems exist which provide real-time monitoring of fracture growth during hydraulic fracturing treatment. Such known systems pump fracturing fluid to the point of injection, provide a radioactive tracer at the point of injection by activating the fracturing fluid and/or proppant using a neutron source or by explosive injection of conventional tracer material, and then monitor the propagation of the radioactive fracturing fluid and/or proppant during the fracturing process by employing a plurality of gamma ray detectors, with conventional signal processing of the spectral data, positioned above and below the point of injection.
Such systems, while providing real-time monitoring of fracture growth during a hydraulic fracturing treatment do not distinguish between tracer material within the borehole versus tracer material within the formation itself.
The present invention is directed to improving upon the known techniques and systems described above by providing a new method for monitoring the hydraulic fracturing of a producing subterranean formation. The method may be utilized to control the fracturing operation; for example, by extending or reducing the injection period as a function of monitored parameters in the subterranean zone of interest.